Measuring your mass
A READER emailed me recently to take issue with the statistics I’d quoted on obesity in New Zealand – that about 1.5 million of us are technically obese, and another 1.3 million of us are overweight.
My correspondent wanted to know what these numbers were based on. ‘‘I bet my bottom dollar it was the BMI [body mass index]!’’ she said.
‘‘As you would already know (but not many in the general public would), it is an extremely blunt tool for measuring obesity. It does not take into account body composition at all.’’
She’s right about that. Body mass index is the measure of your fatness based on your weight divided by your height squared (BMI = kg/m2).
It is best used as a screening tool to identify whether or not a person might have a weight-related health issue. It’s also useful at a population level to measure overall levels of obesity, as it does in the figures I quoted. (And there’s no doubt that at a population level, we are getting fatter.)
BMI is less reliable at an individual level. People often comment to that effect – the example used is of the All Blacks, some of whom would be technically obese based on their BMI alone.
However, while it is definitely possible to be overweight and still fit and healthy (and this is arguably better than being slim and unfit), I’d still suggest that of the people classified as overweight or obese, few would fall into the ‘‘All Blacks’’ category: Highly athletic, lean individuals whose weight and height erroneously gives them an overweight BMI.
That said, there are other ways to measure health which are potentially more useful.
One is waist circumference. It’s an easy DIY test: Measure your waist halfway between the bottom of the ribs and the top of the hips (usually around the belly button). Again, there’s individual variation, but the main point is that your health risk increases as your girth increases.
For men, a waist circumference of 94cm or more is considered higher risk, with over 102cm considered substantial risk. For women, a waist over 80cm is an increased risk, and over 88cm is high risk.
Another measure is waist-toheight ratio. It’s easy to remember the guideline: Your waist circumference should not exceed half of your height.
This applies to both men and women. So for example, if you are 170cm tall, your waist circumference should be no more than 85cm.
Something else that’s worth measuring, although it’s a little more difficult, is body composition – in other words how much fat and muscle you have.
We all potentially lose muscle and gain fat as we get older, and this can be dangerous, whether or not we’re overweight.
Gyms and some medical practices have equipment to measure this. And your doctor can measure your blood pressure, blood cholesterol, triglycerides and blood sugar. All of these together will give a snapshot of your health. As with healthy eating, we need to look at the big picture, not just one element.